Processes for the conversion of coal and other hydrocarbons such as natural gas to a gaseous mixture consisting essentially of hydrogen and carbon monoxide, or of hydrogen and carbon dioxide, or of hydrogen and carbon monoxide and carbon dioxide, are well known. Although various processes may be employed for the gasification, those of major importance depend either on the partial combustion of the fuel with an oxygen-containing gas or on the high temperature reaction of the fuel with steam, or on a combination of these two reactions. An excellent summary of the art of gas manufacture is given in Encyclopedia of Chemical Technology, Edited by Kirk-Othmer, Second Edition, Volume 10, pages 353-433, (1966), Interscience Publishers, New York, N.Y., the contents of which are herein incorporated by reference. The techniques for gasification of coal or other solid, liquid or gaseous fuel are not considered to be per se inventive here.
It is also well known that synthesis gas will undergo conversion to reduction products of carbon monoxide, such as hydrocarbons, at from about 300.degree. F. to about 850.degree. F., under from about one to one thousand atmospheres pressure, over a fairly wide variety of catalysts. The Fischer-Tropsch process, for example, which has been most extensively studied, produces a range of liquid hydrocarbons, a portion of which have been used as low octane gasoline. Catalysts that have been studied for this and related processes include those based on iron, cobalt, nickel, ruthenium, thorium, rhodium and osmium, or their oxides.
As can well be appreciated, the patent and technical literature relating to the Fischer-Tropsch process, is, indeed, extensive and the various catalysts reported in the prior art have been used by themselves as well as in admixture with catalytically inactive supports such as kieselguhr. Although the reasons for using catalytically inactive supports have varied, nevertheless, it would appear that one reason for using the same was that it resulted in increased surface area of the Fischer-Tropsch component upon which it was deposited or admixed and that it also aided in controlling the heat requirements of the overall exothermic reactions.
The wide range of catalysts and catalyst modifications disclosed in the art and an equally wide range of conversion conditions for the reduction of carbon monoxide by hydrogen provide flexibility toward obtaining selected boiling-range products. Nonetheless, these conversions still leave much to be desired because either the catalyst is costly or by-products are produced in excessive amount. A review of the status of this art is given in "Carbon Monoxide-Hydrogen Reactions", Encyclopedia of Chemical Technology, Edited by Kirk-Othmer, Second Edition, Volume 4, pp. 446-488, Interscience Publishers, New York, N.Y., the text of which is incorporated herein by reference.
Iron catalysts have been extensively studied and used in the Fischer-Tropsch process. In general, they are inexpensive and exhibit good activity, and they contain potassium promoter which serves to control the amount of methane plus ethane by-product. The molecular weight distribution of the product is controlled to a great extent by the nature of the reaction, and it has come to be recognized that this follows the Schulz-Flory distribution. See, e.g., P. Biloen and W. M. H. Sachtler, Advance in Catalysis, Vol. 30, pp. 169-171 (Academic Press, New York, N.Y., 1981), which is herein incorporated by reference. Thus, reduction of the methane plus ethane make, designated herein also as the C.sub.1 +C.sub.2 .degree. by-product, although desirable for increasing the total liquid yield, still leaves unsolved the problem of how to increase the amount of liquid product in the boiling range of gasoline and diesel fuel, for example. In recent years it has become known to modify the Fischer-Tropsch liquid products by the conjoint use of a zeolite catalyst exemplified by HZSM-5 and modifications thereof. U.S. Pat. No. 4,086,262 to Chang et al describes the method wherein an intimate mixture of an inorganic carbon monoxide reduction catalyst and certain acidic crystalline aluminosilicate is used as catalyst for producing hydrocarbons from synthesis gas. The two components may be contained in the same or in different catalyst particles. The entire content of this patent is incorporated herein by reference as if fully set forth.
Although the patent just described represents a distinct advance in control of both product distribution and the nature of the products obtained with iron catalysts, the potassium promoter and other constituents such as nitrogen compounds that may be present in the conventional iron catalyst tend to interact with the zeolite in such a manner as to reduce the effectiveness of the catalytic mixture.
It is an object of this invention to provide a novel intimate mixture of an inorganic, potassium promoted, precipitated iron catalyst and a selected zeolite, which mixture is highly selective for liquid fuel products. It is a further object to provide a zeolite-modified Fischer-Tropsch type catalyst that has a low content of nitrogen. It is a still further object to provide an improved process for the conversion of synthesis gas to gasoline distillate fuel in the diesel boiling range. These and other objects of this invention will become evident to one skilled in the art on reading this entire specification including the appended claims.